The present invention relates generally to the art of electroforming, and more particularly to the art of electroforming a grid which provides shielding from electromagnetic pulse (EMP) effects.
Electroforming of precision patterns, such as those used in optical systems, has been accomplished by several methods. For example, precision mesh patterns have been produced by electroplating onto a master pattern of lines formed by etching or ruling lines into a glass substrate and depositing a conductive material into the etched or ruled lines to form a conductive master pattern for electroplating. A major disadvantage of this method is the limitation on the fineness and precision of etching glass.
Photolithographic techniques have also been used to produce patterned electroforming mandrels. For example, a conductive substrate, such as a polished stainless steel plate, is coated with a layer of photoresist. A patterned photomask is placed over the photoresist, which is then exposed to actinic radiation through the mask, thereby creating a pattern of exposed and unexposed photoresist which is further developed. Either the exposed or the unexposed portions of the photoresist are removed, depending on whether a positive or negative pattern is desired, resulting in a conductive pattern on the substrate. An electroplating process is then carried out to form a replica of the conductive pattern which can thereafter be removed from the substrate.
U.S. Pat. No. 3,703,450 to Bakewell discloses a method of fabricating precision conductive mesh pattern on a repetitively reusable master plate comprising a conductive pattern formed on a nonconductive substrate and a nonconductive pattern formed in the interstices of the conductive pattern. A reproduction of the master pattern is formed by plating of a conductive pattern onto the master pattern within a matrix defined by the nonconductive pattern. The conductive metal master pattern is typically deposited onto a glass plate by evaporation of a metal such as chromium through a ruled pattern formed on a stencil material. The nonconductive pattern is formed by depositing a layer of photoresist over the conductive pattern coated side of the glass plate. By exposing the photoresist to actinic radiation through the conductive pattern coated substrate, exact registration of the conductive and nonconductive patterns is achieved. The photoresist layer is developed and the exposed portions are removed, leaving a pattern of photoresist over the conductive pattern. A silicon monoxide layer is then deposited over the entire surface of the glass plate, covering both the photoresist/conductive pattern coated portions and the exposed glass portions. Finally, the photoresist overlying the conductive pattern and the silicon monoxide overlying the residual photoresist material are removed, leaving the glass plate coated with a conductive metal pattern and an array of silicon monoxide deposits in the interstitial spaces in the conductive pattern. Replicas of the conductive pattern are then formed by electroplating.
U.S. Pat. No. 3,833,482 to Jacobus discloses a matrix for the formation of fine mesh comprising a base plate, a photoresist defining the mesh pattern, and a silica coating encapsulating the top of the base plate and the photoresist. A layer of electrically conductive metal is sputtered over the entire surface of the matrix, followed by removal of the conductive metal from the top surface of the resist on the matrix. The matrix is then suitable for electroforming on the layer of conductive metal located in the recess of the matrix.
U.S. Pat. No. 3,878,061 to Feldstein discloses a matrix comprising a highly polished, degenerately doped silicon single crystal substrate having a layer of inorganic dielectric thereon and a pattern of grooves in the dielectric coating exposing the silicon surface.
"A New and Unique Element for Aircraft Transparencies" by Olson et al from the Conference on Aerospace Transparent Materials and Enclosures, December 1983, describes an element comprised of myriad thin filaments prepared by a photolithographic/chemical processing method which involves generating a master pattern, producing a photomask of the pattern, applying a conductive metal layer over a substrate, coating the metal layer with photoresist, exposing the photoresist through the photomask, developing the photoresist, and placing the substrate in an etchant to remove the unwanted material leaving only the desired pattern, which functions as a heating element.
Grids for EMP and microwave attenuation have been used in special purpose aircraft transparencies for a number of years, since it is imperative that electronic systems essential to national security function properly during and after exposure to a nuclear environment. A characteristic of the hostile nuclear environment is the multiplicity of destructive mechanisms; an electromagnetic pulse (EMP) is only one of many products of a nuclear detonation. During a nuclear event, the gamma rays from the burst collide with air molecules in the atmosphere creating Compton electrons which move rapidly away from the center of the burst. This large-scale separation of charges creates a strong nonradiated electric field between the electrons and the parent ions. The movement of these charges produces a Compton current. Most of the EMP energy lies between 10 kHz and 100 mHz, and the pulse is characterized by electromagnetic fields with short rise times (a few nanoseconds) and a high peak electric field amplitude (50 kilovolts per meter). A critical property of EMP is its devastating range; if a high-yield EMP weapon is detonated above the atmosphere, EMP has the capability of disabling electric and electronic systems as far as several thousand miles from the detonation site. EMP grids fabricated by chemical machining have been acceptable for attenuating EMP. However, such coarse-line orthogonal patterns have been nonuniform, especially in cross-section. A fine-line orthogonal grid fabricated by an electroforming process in accordance with the present invention has improved optics and shielding characteristics with a uniform nearly square cross-section.
Electrically, small apertures on an electromagnetic shield are best characterized in terms of magnetic and electric field polarizabilities. An electrically small aperture can be defined as having dimensions significantly less than a wavelength at the highest frequency of interest. In the case of EMP, this frequency is on the order of 100 mHz, corresponding to a wavelength of three meters. Thus, apertures of about one-half meter could be considered reasonably small. Since a typical aircraft transparency has dimensions on this order, the application of a grid to the transparency can provide EMP shielding without substantially compromising visibility.
Polarizabilities of an aperture are the quantities which relate the external incident fields to the equivalent dipole moments for the electric and magnetic fields inside the aperture. Since the polarizabilities depend only on the size and shape of the aperture, they can be used to define a complete electromagnetic description of the aperture. The polarizabilities are found to vary as the cube of the aperture diameter. The term "normalized polarizabilities" is used to describe the attenuation effect of placing a thin film or metallic grid over an aperture. Normalized polarizability, a.sub.n, is defined as the ratio of the polarizability of the shielded aperture, a, to the polarizability of the open aperture, a.sub.o. The "shielding effectiveness" of an EMP shielded transparency is directly dependent on the normalized polarizability as shown in the following equation: EQU 20 log(a/a.sub.o)=20 log a.sub.n.
Since the electroforming method of the present invention produces finer lines of more uniform cross-section, a greater grid density is possible, i.e., more apertures in a given surface area. Since dividing an aperture into N apertures reduces the penetration field by 1/ N, the shielding effectiveness is improved by increasing the grid density with the finer grid lines provided by the method of the present invention. Moreover, the nonorthogonal grid patterns of the present invention provide improved optical properties and greater physical flexibility to conform to compound curves and complex shapes.